1. Field of the Invention:
This invention relates to digital-to-analog conversion circuits and particularly to digital-to-analog conversion circuits using charge coupled devices.
2. Description of the Prior Art:
There are several well-known types of conventional digital-to-analog (D/A) converters that have been used or proposed for various applications. One type includes a circuit in which a plurality of capacitors are charged by digitally weighted voltages. In one embodiment, the capacitors are then placed in series to provide an analog output voltage. In a particular embodiment of this type of D/A converter circuit, resistors are used to provide the digitally weighted voltages. However, a disadvantage of this type of D/A converter is that in a discrete implementation, assembly of the individual component parts into a unitary circuit is required. This discrete assembly is costly and time consuming to manufacture. Additionally, the circuits employing discrete elements are subject to more frequent failures and relatively high power consumption making them undesirable in applications requiring high reliability and low maintenance. Another disadvantage is that in an integrated implementation, resistors are difficult to construct because values must be precisely selected. Furthermore, the use of resistors increase device power consumption.
Charge storage devices other than capacitors can be used to implement D/A converters, for example, charge transfer devices (CTD) such as charge coupled devices (CCD) and bucket brigade devices (BBD). A primary advantage of CTD is their inherent simplicity and cost effectiveness. The CCD, for example, is very basic in structure, being defined substantially by a homogeneously doped substrate with potential wells formed therein, p-n junctions being required only at the input and output. This construction requires much less area than fabrication of an equivalent capacitor circuit, for example, with the accompanying benefits of greater yield, less cost, and, of course, smaller devices.